1. Field of the Invention
This invention relates to charged particle beam recording on film and more specifically to a system for accurately tracing with an electron beam on an electron sensitive film. Such a system finds use in recording graphic images where a high degree of resolution and accuracy of placement is needed. Thus, electron beam recording is highly useful for, among other things, television recording, computer output microfilm, mass data storage, wide bandwidth signal recording, satellite photography, automated cartography, computer micrographics and seismic recording.
2. Description of the Prior Art
Essential elements of an electron beam recorder (EBR) are an electron gun, an electro-optical focusing and deflection system, an electron sensitive film and film gate. In operation, the electron beam traces an image across the recording area on the emulsion side of a stationary film in the film gate. Because scattering and attenuation of the electron beam occurs in air, electron beam recording must be conducted in a high vacuum. Generally, a three (3) stage vacuum system is used in electron beam recorders with the highest degree of vacuum in the vicinity of the electron gun, a lower degree of vacuum in the vicinity of the emulsion side of the film and the lowest degree of vacuum in the film transport chamber.
The vacuum system of the charged particle beam recorder is a high performance, fully automatic, three (3) stage, differentially pumped system which ensures that proper vacuum is maintained in every section of the beam recorder.
Typically, the electron gun chamber, stage 1, is operated at 10.sup.-7 to 10.sup.-6 Torr, the electron optics chamber, stage 2, at about 10.sup.-4 Torr and the film chamber, stage 3, at about 100 millitorr.
The vacuum system uses two diffusion pumps to provide the required degree of vacuum in stages 1 and 2. These diffusion pumps have very low oil back-streaming rates and in addition, are provided with optically dense, high conductance baffles to minimize oil contamination problems. The baffle for the diffusion pump connected to the electron gun chamber is cooled thermoelectrically.
Three direct drive mechanical pumps are also used in the vacuum pumping system of the electron beam recorder. Two of these pumps, are used as backing pumps for the diffusion pumps of stage 1 and stage 2, separately. The third, mechanical pump, is connected directly to the film chamber (stage 3).
A good vacuum system is essential for a long operating life of the cathode of the electron gun. Typically the electron gun used in the beam recorder is a triode with a directly heated tungsten emitter. The aperture in the grid cup of the electron gun is small resulting in a less divergent beam and consequently in improved resolution. Also the aperature in the anode is small and is sensitive to contamination arising from the pumping system.
The cathode (directly heated therminonic emitter) of the electron gun is a TYPE AR filiament used for applications where the ultimate stability is demanded and long operating life is important. Typically the element is of a tungsten-rhenium alloy to ensure ultimate performance and has a mean time between failures of thousands of hours, provided that it is operated in an adequately high vacuum. While the cost of replacement filaments is relatively inexpensive, the service costs for installation of replacement filaments is considerable. Moreover, replacing filaments results in recorder shutdown time and further loses incurred thereby. These factors make extension of filament operating life important.
Obviously, therefore, if one were able to achieve an equivalent or better three stage vacuum system in a more efficient manner, a substantial savings would be realized in costs of pump means, shutdown loses and installation service charges. A need has then been shown to exist in the prior art for a new and improved vacuum system for a charged particle beam recorder.
One technique for solving the problem which has been employed was to provide a three (3) stage vacuum system using a single mechanical pump as a backing pump to both diffusion pumps. This arrangement, however, proved to be unacceptable as back pressures were created by the common feed from the discharge of the two diffusion pumps into the mechanical pump. As a result the pressure reduction to the levels sought for the first vacuum stage was not obtained.